This invention relates to a pressure fixing device in which a recording sheet bearing thereon an image formed of toner is passed between a pair of pressure rolls so that the toner is crushed for fixation.
As shown in FIGS. 1 and 2, a prior art pressure fixing device of this type comprises a lower housing 12 fixed on a base 11, and a swingable upper housing 16 one end of which is pivotally connected to one end of the lower housing 12 by means of a pin 15. A lower roll 14 is rotatably supported on the lower housing 12 by means of bearings 13 fixed thereto, while an upper roll 18 is supported on the upper housing 16 by means of bearings 17 fixed thereto. The basal part of a bolt 19 is fixed to the other end of the lower housing 12. The bolt 19 penetrates a hole at the other end of the upper housing 16 so that the tip end of the bolt 19 extends upward from the upper housing 16. A nut 21 is screwed on the upper end portion of the bolt 19, and a compression coil spring 20 is interposed between the nut 21 and the upper housing 16 so as to surround the bolt 19. The spring 20, the bolt 19, and the nut 21 constitute a pressure mechanism. The lower and upper rolls 14 and 17 can be pressed against each other through the medium of the spring 20 and the housings 12 and 16 under a necessary pressure for fixation as the nut 21 is tightened.
The prior art pressure fixing device of the aforementioned construction generally requires high inter-roll pressure and hence, the use of rolls with a large diameter. Accordingly, the gross weight of the fixing device is substantial, and the pressure mechanism must be projected from the upper housing, extending at right angles to the course of recording paper. Thus, the pressure mechanism directly adds to the external dimensions of the fixing device, resulting in an increase in setting space for the fixing device which may be incorporated in a facsimile system or copying apparatus.
In consideration of these circumstances, there is conventionally proposed a pressure fixing device with a construction as shown in FIGS. 3 and 4. This device has a pair of frames 32 which are fixed by prism members 31, spaced apart and facing each other. Each frame 32 is provided with a bearing 33 fixed thereto, and bearings 37 and 38 supported by two guide members 35 and 36, respectively, which can move vertically. An upper roll 34 is rotatably supported by the fixed bearings 33, while intermediate and lower rolls 39 and 40 are rotatably supported by the intermediate and lower movable bearings 37 and 38, respectively. The tip end of a pressure bolt 41 is located under the lower guide member 36 so as to face the same. The bolt 41 is screwed into its corresponding frame 32 from the underside thereof so that it can be adjusted from under the frame 32. A Belleville spring 42 is interposed between the guide member 36 and the tip end of the bolt 41. Pushed up by the bolt 41, the Belleville spring 42 urges the guide member 36 upward. Thus, the lower roll 40 supported by the bearings 38 is urged upward by the guide members 36 to press the intermediate roll 39 against the upper roll 34 with a given force.
Using three rolls with small diameters, the pressure fixing device shown in FIGS. 3 and 4 can enjoy relatively compact, lightweight design. The pressure between the rolls is produced and adjusted by pushing the Belleville springs 42 from under the frames 32 by means of their corresponding pressure bolts 41 screwed in the frames 32 to adjust the flexure of the springs 42. Since the Belleville spring 42 has a spring constant much greater than that of the compression coil spring 20 used in the first example, it is difficult and requires skill to set the pressure to a given level. Since the pressure bolts 41 for pressure adjustment must be handled from under the frames 32, it is guide difficult, if not impossible, to adjust the pressure where the fixing device is incorporated in a facsimile system or copying apparatus.
In the prior art pressure fixing devices of the aforementioned types, the rolls are heavily pressed against one another in a stop or no-load operation mode. Therefore, the working life of the rolls, as well as that of the bearings which are continually subjected to high pressure, is relatively short. Since the rolls are pressed against one another under high pressure, the recording paper to be fixed cannot be easily and smoothly inserted between the rolls, possibly causing a jam. When the recording paper leaves the rolls, moreover, the rolls will clash with each other to produce a noise.